The invention relates to a method of measuring properties of paper, the method comprising measuring properties of paper with an apparatus comprising at least one measuring probe having a feed hole substantially symmetrically placed in the center of the measuring probe, medium being fed between the measuring probe and the paper via the feed hole.
The invention further relates to an arrangement in a paper measuring apparatus, the arrangement comprising at least one measuring probe, a sensor arranged in connection with the measuring probe, a feed hole arranged substantially symmetrically in the center of the measuring probe, and means for feeding medium between the measuring probe and the paper through the feed hole.
In a paper machine, the thickness and other properties of paper and cardboard are typically continuously monitored as the paper is moving. Apparatuses are usually fastened to what is known as a measuring beam, in which the apparatuses continuously move in a reciprocating manner in the transverse direction of the paper and continuously measure the paper as it moves forward. Sensors are arranged as close to the surface of the paper as possible to improve measuring accuracy. This causes a plurality of problems, e.g. if the measuring sensors for some reason come into contact with the surface of the paper, the sensors may leave tracks on the paper. Furthermore, the contact may cause fouling of measuring heads, resulting in impaired measuring accuracy in the long run. The contact of the sensors with the irregularities of the surface of the paper may also cause roles to be formed in the paper. Consequently it is known to use what is known as on air bearing, the sensors being provided with a bored hole or holes through which air is blown between the sensor and the paper web. Owing to the air bearing, the sensor is able to stay away from the web, and hence e.g. irregularities on the surface of the paper do not come into contact with the measuring head. This avoids formation of holes in the paper. Formation of traces on the surface of the paper is also avoided and fouling of the sensors diminishes. Such air bearing solutions are disclosed in e.g. German Offenlegungsschrift 28 29 264 and 40 11 646 and in U.S. Pat. Nos. 4,450,404 and 4,528,507 and in WO 95/30877. It is common to all these solutions that air is blown either from a plurality of apertures or at a high pressure or from a large area or in some other way, ensuring that the quantity of air keeps the measuring head away from the surface of the paper. However, said solution does not ensure at a sufficient accuracy that the gap between the measuring head and the paper remains constant, and thus the measuring accuracy does not remain sufficiently high. U.S. Pat. No. 3,855,524 discloses a solution corresponding to the above publications, with the exception that air jets are directed inclined towards the edges of the sensor to prevent the sensor from tilting. However, the problems with measuring accuracy are identical to those in the above publications.
WO 96/35112 discloses a sensor provided with an asymmetrically arranged opening from which air is blown between the sensor and the paper. Around the opening, close to it, is a circular channel into which the air between the sensor and the paper flows through the opening. The circular channel is in communication with a discharge channel via which the air blown to the circular channel is led out. Other means are used to blow air into the discharge channel to provide an ejector effect to increase the stream velocity of the air flowing in the circular channel. An increase in the stream velocity reduces the pressure in the portion inside the circular channel below ambient. The solution is extremely complex and the flow resistances can easily cause failure of achieving the underpressure. The apparatus is difficult to manufacture, and e.g. the discharge channel can absorb dust, making the apparatus extremely difficult to maintain.
It is an object of the present invention to provide an arrangement which avoids the above drawbacks.
The method of the intention is characterized in that medium is fed to produce a force which keeps the measuring probe at a distance from the surface of the paper, the diameter of the feed hole, the diameter of the measuring probe, the distance between the measuring probe and the paper, and the pressure for feeding the medium being arranged such that when the medium is discharged from the feed hole, the flow rate of the medium increases so that the flow produces underpressure which generates a force between the paper and the measuring probe, which force tends to move the measuring probe and the paper towards one another, whereby they are set in a state of equilibrium determined by said forces.
The arrangement of the invention is characterized in that at a given pressure for feeding medium the diameter of the feed hole, the diameter of the measuring probe and the distance between the measuring probe and the paper are arranged such that when the medium is discharged from the feed hole, the flow rate increases so that the flow produces underpressure which generates a force between the paper and the measuring probe, which force tends to move the measuring probe and the paper towards one another, thus keeping them close to one another, whereby the measuring probe and the paper are set in a state of equilibrium with respect to their mutual distance.
It is an essential idea of the invention that the measuring probe of the measuring apparatus is kept at a distance from the surface of the paper by a feeding medium, preferably air, through the measuring probe, between the measuring probe and the paper so as to adjust the measuring probe and the paper in a state of equilibrium with respect to their mutual distance. It is a further idea that the pressure of the medium to be fed, the dimensions of the feed hole and the measuring probe and their distance from the surface of the paper are arranged in such a way that after the medium is discharged from the feed hole, its flow rate increases so as to produce underpressure which generates a force between the paper and the measuring probe, which force tends to move them towards one another. On the other hand, the feeding of the medium generates a force between the measuring probe and the paper forcing them away from one another, whereby the measuring probe and the paper settle in a state of equilibrium determined by said forces.
It is an advantage of the invention that the measuring probe and the paper can be set in such a state of equilibrium that the distance between them is extremely accurate and remains constant. Furthermore, a gap remains between the measuring probe and the paper, whereby the irregularities on the surface of the paper do not come into contact with the measuring head, and holes are not formed in the paper. Traces are not formed on the surface of the paper and fouling of the sensor can be kept relatively slight.
In addition to paper, the term paper refers herein also to cardboard.